EC4304B Datasheet, PDF (7/11 Pages) E-CMOS Corporation – PWM Control 1.2A Step-Down Converter

PWM Control 1.2A Step-Down Converter EC4304B
Driving the ADJ input via open collector transistor
The recommended method of driving the ADJ pin and controlling the amplitude of the PWM
waveform is to use a small NPN switching transistor as shown below:
PWM
ADJ
EC4304B
GND
This scheme uses the 200k resistor between the ADJ pin and the internal voltage reference as
a pull-up resistor for the external transistor.
Driving the ADJ input from a microcontroller
Another possibility is to drive the device from the open drain output of a microcontroller. The
diagram below shows one method of doing this:
MCU
ADJ
EC4304B
GND
If the NMOS transistor within the microcontroller has high Drain / Source capacitance, this
arrangement can inject a negative spike into ADJ input of the EC4304B and cause erratic
operation but the addition of a Schottky clamp diode (cathode to ADJ) to ground and inclusion of a
series resistor (10K) will prevent this. See the section on PWM dimming for more details of the
various modes of control using high frequency and low frequency PWM signals.
Shutdown mode
Taking the EN pin to a voltage below 0.8V for more than approximately 100us, will turn off the
output and supply current will fall to a low standby level of 35uA nominal.
E-CMOS Corp. (www.ecmos.com.tw)
Page 7 of 11
2011/10/06